In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the change thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The tone powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. The copy sheets are collected and bound or stapled together into sets of copy sheets. The bound or stapled sets of copy sheets are then stacked for presentation to the machine operator.
In commercial high speed printing machines of the foregoing type, large volumes of sets of copy sheets are fed onto a stacking tray. When the tray is loaded to its capacity, an elevator moves the tray to a station where an operator can readily remove the sets of copy sheets. Frequently, the printing machine is idling and not producing copy sets while the operator is unloading the previously completed sets from the stacker tray. This reduces the productivity time of the printing machine by increasing its down time. Ideally, high capacity printing machines should be run on a continuous basis and the unloading of copy sets should be such that the operator can simply and easily remove copy sheet sets from one sheet stacking apparatus while a new batch of copy sheet sets are being run into a second sheet stacking device. However, presently, most high speed printers use a single elevator manuevered tray for receiving copy sheet sets, which is cumbersome for copy set removal, or use a single container and a pedestal to unlead copy sheet sets, for example, the Xerox.RTM. 9700 printer. Also, previous high speed printers handled 81/2.times.11" and 14" sheets with and without containers. Accordingly, it is desirable for printing machines to have unloading while run capability and to be able to handle all sizes of copy sheets and all sizes of containers from B5 to A3 with ease.
Various approaches have been devised for stacking and unloading sets of copy sheets. The following disclosures appear to be relevant:
U.S. Pat. No. 3,747,920; Patentee: Linkus; Issued: Jul. 24, 1973 PA1 U.S. Pat. No. 4,359,218; Patentee: Karis; Issued: Nov. 16,1982 PA1 U.S. Pat. No. 4,423,995; Patentee: Karis; Issued: Jan. 3, 1984 PA1 U.S. Pat. No. 4,477,218; Patentee: Bean; Issued: Oct. 16, 1984 PA1 U.S. Pat. No. 4,479,641; Patentee: Bean et al.; Issued: Oct. 30, 1984 PA1 U.S. Pat. No. 5,017,972; Patentee: Daughton et al.; Issued: May 21, 1991 PA1 U.S. Pat. No. 5,018,717; Patentee: Sadwick et al.; Issued: May 28, 1991
The relevant portions of the foregoing patents may be summarized as follows:
Linkus discloses a sheet unloading apparatus used in conjunction with a punch press. A trolley moves material from a loading position to an unloading position. A support table receives sheets from the trolley and is vertically movable by a motor operated scissors type of support.
Karis (U.S. Pat. No. '218) describes a sheet collection and discharge system. Sheets continuously accumulate at a stacker station. A table supported for vertical movement on scissor type collapsible legs receives the sheets. The lower ends of the legs have rollers for transversing the apparatus across linear tracks. The table has a base platform element, the under surface of which is formed with connection pieces to which the upper ends of the support legs are attached. A series of spaced apart columns extend vertically from the upper surface of the table platform. Each column is generally rectangular with a longitudinal axis parallel to the longitudinal axis of the apparatus. The upper surfaces of the columns support the stack of sheets at the stacker station. Interspaced between the table carrying columns are a series of lateral belt conveyors driven by a motor through a series of rollers. The belt conveyors discharge sheets in a batch onto a discharge table surface after the upper carrying surfaces of the table have descended beneath the level of the conveyor belts.
Karis (U.S. Pat. No. '995) discloses a continous sheet feeding machine provided with a sheet collection area for receiving and stacking sheets into either ream or skid loadings. Two separate scissor type lift tables and discharging devices are provided for the two types of piling methods. Motor driven screw arrangements shuttle the different lift tables into their proper positions. The ream table has a table base portion secured to the ream collection frame and a vertically movable table top portion on which a ream size pile of sheets can be collected in the collection area. Scissor type lift means are suitably connected between the table base and table top to raise and lower the table top. The table top has a series of parallel, spaced apart platform surfaces which fit in the spaces between the discharge conveyor belts, such that, after a ream pile has accumulated on the table top, the ream pile may be transferred to the discharge conveyor belts by lowering the table top beneath the level of the belts. The conveyor belts than draw the ream pile off the table top.
Bean describes an offset stacker having a frame provided with a tray located therein which is movable between an upper stacking station and a lower discharge station. Movable jogger arms aid in accumulating sets of sheets on the tray in an offset manner at a loading station. The tray is moved down to the discharge station by a pulley device to present stacked materials for removal from the stacker. The tray includes cutouts in registry with rollers so that the rollers may protrude above the tray at the discharge station.
Bean et al. teaches a paper handling system for use with a duplicating machine. Paper sheets are collected into sets and are transported to a finishing station where they are bound into pamphlets. The sheets are then stacked on a tray at a stacking station and moved to a discharge station. A discharge conveyor transports stacked sheets to a shelf for removal. The discharge station includes a discharge conveyor system which consists of a pair of belts which may run from the tray to the end of the discharge station. Rollers located within the stacker, extend upwardly through the tray to displace a stack of pamphlets to the conveyor system.
Daughton et al. discloses an elevator position control apparatus that maintains a copy sheet support surface within an established range in order to uniformly stack copy sheets on the support surface.
Sadwick et as. describes a sheet stacking apparatus which includes a tray that receives sets of copy sheets at a loading station and moves the sets of copy sheets to a discharge station. At the discharge station, the sets of copy sheets are transferred to a drawer. The drawer moves the sets of sheets from a discharge station to an unload station. As the sets of sheets are being unloaded from the drawer, additional sets of sheets are being loaded on the tray.